Direct-current-to-alternating-current (DC-to-AC) inverters are widely used in portable systems, uninterruptible power supplies, and the like. There are uses for converting variable-frequency AC voltages, such as obtained from a variable-speed engine (as, for example, where the engine of an airplane rotates at a speed determined by the aircraft's airspeed, and its alternator also rotates at a corresponding speed) to fixed-frequency AC voltages, which may be single-phase or polyphase AC. In such cases, conversion is generally realized by first rectifying the variable-frequency AC to a DC voltage, and then inverting the DC into a fixed-frequency AC with a static inverter. Such an arrangement for a Variable Speed Constant Frequency (VSCF) drive has substantial advantages over mechanical constant-speed drives or rotating motor-generator sets. The output voltage of such a VSCF may be a three-phase output, where the load can be either a single 3-phase load or multiple single-phase loads (although with multiple independent single-phase loads the like-numbered independent single-phase currents can not be guaranteed to be balanced and the neutral connection must be capable of supplying any resulting difference currents).
FIG. 1 is a simplified diagram of a prior-art AC-to-AC inverter 10. In FIG. 1, a polyphase generator 12, illustrated as three separate sources 12a, 12b and 12c, is connected in a wye configuration with three terminals 14a, 14b, and 14c, each of which is respectively connected to the associated diodes or rectifiers of a polyphase rectifier bridge 16. In keeping with the illustration of the polyphase source 12 as being a three-phase source, the rectifier bridge 16 is illustrated as being a 6-pulse rectifier, having a first diode 16a with its anode connected to AC terminal 14a and its cathode connected to pulsating direct voltage terminal or bus 18a, and a second diode 16b having its cathode connected to AC terminal 14a and its anode connected to pulsating direct voltage bus or terminal 18b. Rectifier bridge circuit 16 also includes: a third diode 16c having its anode connected to AC terminal 14b and its cathode connected to pulsating direct voltage terminal or bus 18a; a fourth diode 16d having its cathode connected to AC terminal 14b and its anode connected to pulsating direct voltage bus or terminal 18b; a fifth diode 16e having its anode connected to AC terminal 14c and its cathode connected to pulsating direct voltage terminal or bus 18a; and a diode 16f having its cathode connected to AC terminal 14b and its anode connected to pulsating direct voltage bus or terminal lab.
The DC voltage produced across bus terminals 18a and 18b by rectifier 16 is connected between a positive input bus 22a and a negative input bus 22b of a DC-to-AC inverter 40. A polyphase switching circuit, illustrated as a three-phase circuit including first, second, and third switching half-bridges, is coupled across conductors 22a and 22b of the direct-voltage bus. The first switching half-bridge includes series-connected controlled switches 40a and 40b connected across conductors 22a and 22b, with a first output voltage terminal 42a at the juncture of the switches 40a and 40b. The switches are controlled in a complementary manner, by a controller 41, to connect the output terminal 42a to either the positive DC voltage bus 22a or the negative DC voltage bus 22b. The second switching half-bridge includes series-connected controlled switches 40c and 40d connected across conductors 22a and 22b, with a second output voltage terminal 42b at the juncture of switches 40c and 40d. These switches are also controlled in a complementary manner, by controller 41, to connect the output terminal 42b to either the positive DC voltage bus 22a or the negative DC voltage bus 22b. Finally, the third switching half-bridge includes series-connected controlled switches 40e and 40f connected across conductors 22a and 22b, with a third output voltage terminal 42c at the juncture of the switches 40e and 40f. These switches are also controlled in a complementary manner, by controller 41, to connect the output terminal 42c to either the positive DC voltage bus 22a or the negative DC voltage bus 22b.
While the instantaneous voltage at any one of the output terminals 42 is always either the positive DC voltage on bus 22a or the negative DC voltage on bus 22b, the average voltage at that terminal can be anywhere in between these two values, depending on the switch-closure pulse widths. The switching frequency of switching bridge 40 is at a substantially higher frequency than the desired ac output voltage. Output filter 30 connects the output of the switching bridge 40 to the load 50 and averages the pulse widths to recover this value. Switching half-bridge terminal 42a is connected through an inductor 30a to a capacitor 30d and to load 50a, while switching half-bridge terminal 42b is connected through a different inductor 30b to a second capacitor 30e and to load 50b, with switching half-bridge terminal 42c being connected through another inductor 30c to a third capacitor 30f and to load 50c. The load impedances are connected together at a neutral terminal or line 32, along with the end of each of capacitors 30d, 30e and 30f not connected to one of terminals 42. Neutral line 32 is connected to a tap 26 on a capacitor half-bridge 24, which includes a pair of capacitors 24a and 24b connected in series across the direct voltage buses 22a and 22b. The series connection of capacitors 24a and 24b provides a conduction path for the flow of switching currents from switching inverter 40, with the connection to midpoint terminal 26 providing a path for the flow of the neutral switching currents and any unbalanced fundamental currents.
In operation of the arrangement of FIG. 1, polyphase alternator 12 produces alternating voltage, at a frequency proportional to the speed of the rotor. This voltage is rectified by rectifier 16 to produce pulsating direct voltage across terminals 18a and 18b. The switches of switching bridge circuit 40 are controlled by controller 41, as known in the art, to produce pulse-width modulated high-frequency waveforms at AC output terminals 42. Output filter 30 removes the high frequency part and recovers the low frequency ac voltage to the load 50. The method of control of the switches by controller 41 may be of many different methods including sine triangle modulation, hysteretic modulation, sliding mode control, and many other methods as is well known to those skilled in the art. Space vector modulation, with its benefits of increased output voltage, can not be used in this arrangement, since the triplen harmonics generated by space-vector switching cannot be developed at the neutral 32.
The switching of the switches in bridge 40 produces switching frequency currents in capacitors 24. In the arrangement of FIG. 1 capacitors 24a and 24b must be dimensioned not only to filter the high frequency switching currents from switching bridge 40 but also to conduct any low output frequency unbalanced component of load current. The output frequency being substantially lower than the switching frequency and the effect of the impedance of capacitors 24a and 24b being effectively in parallel, causes an output voltage unbalance to be produced by the unbalanced currents; this effect dominates the requirements for dimensioning capacitors 24a and 24b, which are determined, in the main, by the requirement for carrying the unbalanced component of the load current, rather than by the requirement for filtering.
FIG. 2 is a simplified diagram of another prior-art polyphase inverter arrangement. The arrangement of FIG. 2 is identical to that portion of FIG. 1 to the left of terminals 22a and 22b, which portion is illustrated in FIG. 2 as a block designated "as in FIG. 1". In the arrangement of FIG. 2, a single filter capacitor 24 is used instead of the split capacitor 24a/24b as in FIG. 1, because there is no need for a capacitor tap. Instead of a capacitor tap, a further controllable switched half-bridge 240 is coupled across the direct voltage bus 22a, 22b. Switched bridge 240 includes a first controllable switch 240a connected in series with a second controllable switch 240b, with a tap 226 at the juncture of the switches. The neutral of the output is returned via inductor 60 to the tap 226 rather than to a capacitive tap.
In operation of the prior-art arrangement of FIG. 2, the rectified direct voltage is generated across terminals 22a and 22b as described in conjunction with FIG. 1, and the switching bridge 40 operates in the same manner, for switching the filtered direct voltage across terminals 22a and 22b to produce the three-phase output voltages. The modulation of switching bridge 240 is controlled to maintain the average voltage at the neutral point 32 at the exact geometric center of the three output phase voltages. The low-pass filter formed by inductor 60 and capacitors 30d, 30e and 30f average the high frequency switching at tap 226 to recover the average voltage. The arrangement of FIG. 2 may have the switches controlled by space vector modulation, as well as any other suitable method of control. For example, an article entitled "A THREE-PHASE INVERTER WITH A NEUTRAL LEG WITH SPACE VECTOR MODULATION", by Zhang et al., published at PP 857-863 of the Transactions of the 1997 IEEE 12th Applied Power Electronics Conference, ISBN 0-7803-37042, describes how space vector modulation may be applied. The triplen harmonics of the space vector modulation are developed across the neutral inductor 60. This arrangement has the disadvantages of requiring an additional switching bridge and its control means, which adds to the cost and complexity, while reducing reliability. It does, however, allow capacitors 24 to be dimensioned only to aid in filtering of the pulsating direct voltage and to return high-frequency components attributable to the switching.
Improved polyphase inverters are desired.